The field of the disclosure relates generally to solar energy conversion devices, and more particularly to a multi-layer back surface field layer in a solar cell structure.
Interest in photovoltaic (“PV”) cells in both terrestrial and non-terrestrial applications has increased as concerns over pollution and limited resources continue. Regardless of the application of such cells, efforts have been ongoing to increase the output and/or increase the efficiency of PV cells.
Some known PV cell structures include a cell base with layers lattice matched to substrates with relatively high bandgaps. A back surface field (BSF) layer is often used between a cell base portion of a solar cell and the substrate or layers next to the base layer to reduce electron-hole recombination at the interface, block minority carriers from diffusing away from the pn junction and to thereby increase the cell's efficiency. The BSF layer is commonly lattice-matched to the cell lattice constant. The relatively large cell band gaps in such structures, however, reduce the efficiency of the solar cell structure.
Some multijunction terrestrial PV cell structures, utilize an upright metamorphic design with a larger lattice constant and a wider bandgap range to better split the solar spectrum, and therefore to increase efficiency. Some such PV cell structures may have a cell base bandgap range from about 0.7 eV to about 2.0 eV. To lattice match the BSF layer to the larger cell lattice constant in metamorphic PV cell structures, indium is often added to the BSF layer. For example, some known upright metamorphic PV cells use an AlGaInAs or ALGaInP BSF layer. In at least one known metamorphic PV cell structure, the BSF layer includes about twenty percent indium and about sixty percent aluminum. Although the added indium increases the lattice constant of the BSF layer, the indium also reduces the bandgap of the BSF layer. As a result, some known metamorphic PV cell structures have an indirect bandgap below 2.0 eV. At such a relatively low bandgap relative to the cell base bandgap, the BSF layer may not function as effectively as desired, and efficiency of such a PV cell may be reduced. Moreover, p-doping with carbon in the BSF layer is limited because of the indium etching effect of carbon. Thus, at least some known metamorphic PV cell structures that include indium in the BSF layer are limited to p-doping of less than about 1018 cm−3.